Examples of such a conventional apparatus include a printing apparatus configured to perform printing to a printing sheet by moving the printing sheet relative to a printing head only once. Such a printing apparatus is referred to as a one-pass system. When the printing head is provided for color printing, inkjet heads for the same color are arranged in a direction orthogonal to a transportation direction of the printing sheet (i.e., a width direction of the printing sheet), and inkjet heads for different colors are arranged in the transportation direction of the printing sheet. The inkjet heads form the printing head. The printing head having a plurality of inkjet heads for the same color arranged in the direction orthogonal to the transportation direction is referred to as a line head. When the inkjet heads for the same color have different timings of discharging ink droplets, an image to be linear in the direction orthogonal to the transportation direction is shifted in the transportation direction. Such step shift (also referred to as printing shift) occurs. The step shift should be corrected.
The step shift is corrected as under. For instance, the printing sheet subjected to printing is scanned with a scanner of a CIS (Contact Image Sensor) type to obtain a scanned image, and then step shift is determined from the scanned image. Thereafter, a discharge timing of the ink droplets for each of the inkjet heads is controlled in accordance with an amount of determined step shift.
Specifically, a step-shift correcting chart is printed on the printing sheet for determining the amount of step shift. Then, the printing sheet is transported such that the step-shift correcting chart is located within a scanning area of the scanner. The scanner reads the step-shift correcting chart to obtain an image, and analyzes the image to calculate the amount of step shift. See, for example, Japanese Patent No. 4059119.
However, the example of the conventional apparatus with such a construction has the following problems.
Some printing sheet has lateral perforations in a width direction thereof such that a printing area is easily separative. Here, the step-shift correcting chart may be located at the perforation. In such a case, the conventional apparatus cannot distinguish the lateral perforation from the step-shift correcting chart. Accordingly, the apparatus may fail analysis of the amount of step shift with the step-shift correcting chart, causing failure in calculation of the shift amount, or if possible, causing calculation of the shift amount with a low degree of accuracy. Such drawbacks may arise.